Media access control apparatus and method for guaranteeing quality of service in wireless LAN

ABSTRACT

The present invention relates to a media access control (MAC) apparatus and method for guaranteeing quality-of-service in a wireless local area network (LAN). The MAC method comprises: extracting a user priority from a frame received from an upper layer and separately storing a voice frame and a general frame according to an access category (AC); independently performing backoff operations for the voice frame and the general frame; determining whether the backoff operations for the voice frame and the general frame have simultaneously ended; if the backoff operations have simultaneously ended, transmitting the voice frame having a higher priority first and performing the backoff operation for the general frame; and if the backoff operations have not simultaneously ended, transmitting a frame whose backoff operation ends.

BACKGROUND OF THE INVENTION

This application claims the priority of Korean Patent Application No.2003-97155, filed on Dec. 26, 2003, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

1. Field of the Invention

The present invention relates to a quality-of-service technology for acommunication system, and more particularly, to an apparatus and methodfor guaranteeing quality-of-service in a wireless local area network(LAN).

2. Description of the Related Art

A voice-over-Internet protocol (VOIP) technology represents an IPtelecommunication technology used when a plurality of facilitiestransfer voice information using an IP. In general, the VoIP technologydeals with not a conventional protocol based on circuit as used for apublic switched telephone network (PSTN) but a protocol for sendingvoice information with a digital format in discontinuous packets.Therefore, since packet transmission is discontinuously achieved, it isdifficult to guarantee quality-of-service (QoS).

The QoS of voice information must be guaranteed for VoIP services.Accordingly, an IEEE 802.11 wireless LAN media access control (MAC)technology has recently been suggested as a new LAN technology forguaranteeing the QoS.

However, the IEEE 802.11 MAC technology cannot support frames to whichdiscriminated user priorities are applied. Basically, when a channelaccess right is granted, a distributed coordination function (DCF)provides the same proportional channel access right to all stationscontending for channels in a basic service set (BSS). However, the sameproportional channel access right is not preferable for stations havingdifferent user priorities. Therefore, from the point of view of the QoS,the MAC technology must discriminately deal with frames having differentpriorities and provide a QoS field included in a frame header.

To provide these functions, an enhanced distribution coordinationfunction (EDCF) has been included in an IEEE 802.11e standard work. TheEDCF provides a discriminated distribution channel directly to eachframe having 8 user priorities. The 8 user priorities are mapped to 4access categories (ACs), and an IEEE 802.11e station must realize all ofthe 4 ACs.

However, when a terminal for the VoIP is realized, since the terminalcan be sufficiently realized with an AC for a voice frame and an AC fora general use, a method of effectively using the ACs without realizingall the 4 ACs is necessary.

SUMMARY OF THE INVENTION

The present invention provides a media access control (MAC) apparatusfor guaranteeing quality-of-service (QoS) in a wireless local areanetwork (LAN), which can guarantee the QoS of VoIP services with arelatively simple method in a wireless LAN environment, and a methodthereof.

According to an aspect of the present invention, there is provided amedia access control (MAC) apparatus comprising: a first transmissionqueue storing a voice transmission frame; a second transmission queuestoring a general transmission frame; a frame handler extracting userpriority information from a frame input from an upper layer, mapping theframe to a relevant access category (AC), and storing the frame in thefirst transmission queue or the second transmission queue; a MACcontroller determining a backoff operation timing and a frametransmission timing by checking a media status; a first arbitrationinter-frame space (AIFS) timer and a second AIFS timer, each reducing apredetermined timer value set by the MAC controller by a predeterminedvalue unit; a first backoff block and a second backoff block, eachperforming a separate backoff operation for each AC using apredetermined backoff count value; a contention resolution unit, whichtransmits the voice frame having a higher user priority first and givesup transmission of the general frame when two ACs simultaneously end thebackoff operations; a frame detector, which determines whether or not totransmit ACK by checking an ACK policy bit from a header of the receivedframe when a frame is received from a physical layer; and a receivequeue storing the received frame and transmitting the frame to the upperlayer.

According to another aspect of the present invention, there is provideda media access control (MAC) method comprising: extracting a userpriority from a frame received from an upper layer and separatelystoring a voice frame and a general frame according to an accesscategory (AC); independently performing backoff operations for the voiceframe and the general frame; determining whether the backoff operationsfor the voice frame and the general frame have simultaneously ended; ifthe backoff operations have simultaneously ended, transmitting the voiceframe having a higher priority first and performing the backoffoperation for the general frame; and if the backoff operations have notsimultaneously ended, transmitting a frame whose backoff operation ends.

According to another aspect of the present invention, there is provideda media access control (MAC) method comprising: receiving a frame from aphysical layer; decoding an ACK policy from a header of the frame anddetermining whether or not to perform an ACK response based on thedecoded ACK policy; when the ACK response must be performed,transmitting an ACK frame and storing the received frame in a receivequeue; and when a host is ready, transmitting the frame to an upperlayer regardless of a priority.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 illustrates configurations of a header of a MAC frame and a QoScontrol field included in the header of the MAC frame;

FIG. 2 illustrates a procedure for performing an IEEE 802.11 DCF;

FIG. 3 is a block diagram of a MAC apparatus for supporting QoS in awireless LAN according to an exemplary embodiment of the presentinvention; and

FIGS. 4 and 5 are flowcharts illustrating methods of supporting QoS in awireless LAN, which are performed in the MAC apparatus shown in FIG. 3,according to exemplary embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will now be described more fully withreference to the accompanying drawings, in which embodiments of theinvention are shown.

FIG. 1 illustrates configurations of a header of a MAC frame and a QoScontrol field included in the header of the MAC frame. FIG. 2illustrates a procedure for performing an IEEE 802.11 DCF.

An IEEE 802.11 MAC is based on a logic function called a coordinationfunction. The coordination function determines whether a stationtransmits or receives a frame in a basic service set (BSS). Thecoordination function is divided into two functions, such as adistribution coordination function (DCF) based on a contention methodand a point coordination function (PCF) based on a poll-response method,according to a method of obtaining a channel access right. Today, most802.11 apparatuses operate using the DCF.

The 802.11 DCF operates with one transmission queue and is located in adistributed MAC having a local evaluation function with which a channelstatus can be evaluated in order to support a carrier sense multipleaccess collision avoidance (CSMA/CA) protocol.

Referring to FIG. 1, since a QoS control field is included in a headerof a MAC frame, one of a plurality of priority values can be carried inthe QoS control field. If a host or a router sending traffic to a LANgrants an appropriate priority for an individual packet to betransmitted, LAN devices, such as switches, bridges, and hubs,appropriately deal with the packet.

Referring to FIG. 2, if a channel is busy when a frame arrives in atransmission queue from an upper layer, after a MAC apparatus waitsuntil a medium is idle, the MAC apparatus waits during a DCF inter-framespace (DIFS) time. If the channel is still idle after the DIFS timelapses, the MAC apparatus performs a backoff operation (transmissionwait and try) to obtain a channel access right using a random backoffcounter.

If the medium is idle during every slot time, the MAC apparatusdecreases a random backoff count value, and if the count value becomes0, the MAC apparatus transmits the frame. If the transmission queue isempty and the channel is idle for longer than the DIFS time when a framearrives in the transmission queue, the MAC apparatus immediatelytransmits the frame without the backoff operation. If the channelbecomes busy during the backoff operation, the MAC apparatus stops thebackoff operation, and if the channel is continuously idle during theDIFS time, the MAC apparatus performs the backoff operation from thelast backoff count value again.

Each station maintains a contention window (CW), which uses the randombackoff count value. The backoff count value is a pseudo random integerselected with an even probability in a range of [0, CW]. The CW isinitialized to CWmin and increases by CW=2(CW+1)−1 whenever transmissionfails. This is a method for reducing a collision proportion when aplurality of stations try to transmit. The CW is set to at most CWmax,and after frame transmission normally ends, the CW is initialized toCWmin. Also, even if a frame waiting for transmission is not in thetransmission queue, a station, which has transmitted all data, waitsduring the DIFS time, performs the backoff operation, and ends atransmission process.

If a station successfully receives a frame, after a short inter-framespace (SIFS) time lapses, the station indicates that it has received theframe by immediately transmitting an ACK frame. If a station does notreceive the ACK frame after transmitting data, the station performsretransmission after the random backoff operation.

As described above, in the IEEE 802.11 MAC apparatus, if the MACapparatus includes only one transmission queue, since a subsequent framecan be transmitted only after the transmission of a preceding frameends, when the transmission of the preceding frame is delayed, it isdifficult to guarantee QoS. This problem can be solved with a pluralityof queues. The IEEE 802.11e standard recommends more than 4 classes ofqueues in a case of an access point (AP) supporting apoint-to-multipoint access. However, the recommendation is not suitablefor VoIP terminals.

Also, the DCF of the IEEE 802.11 MAC standard uses DIFS, CWmin, andCWmax, in which priorities are not considered. Since the MAC standardperforms the backoff operation during a relatively long time for a framerequiring QoS, it is difficult to guarantee the QoS.

Also, since the DCF of the IEEE 802.11 MAC standard ends a frametransmission process only if an ACK response is received with respect toall data and a management frame, the DCF is not suitable for a framerequiring QoS in which transmission timing is more important thantransmission quality. This problem can be solved by limiting the ACKresponse for a frame for which the QoS is required.

Therefore, in an embodiment of the present invention, to solve a QoSproblem of a terminal supporting a VoIP service with two classes oftransmission queues, for real-time traffic such as an access category 3(AC-3), a queue for VoIP exclusive use is used, an AIFS[3] parameter, aCWmin[3] parameter, and a CWmax[3] parameter are used to guarantee ahigher priority, and an ACK response in response to a transmitted VoIPframe is not received. For general traffic, to support priorities of allframes except the VoIP frame, an AIFS[AC] parameter, a CWmin[AC]parameter, and a CWmax[AC] parameter are used according to the AC, andan ACK response may be received or not. A configuration of a MACapparatus having the features described above will now be described.

FIG. 3 is a block diagram of a MAC apparatus 100 for supporting QoS in awireless LAN according to an exemplary embodiment of the presentinvention. The MAC apparatus 100 is an IEEE 802.11 wireless LAN MACapparatus 100 suitable for a terminal supporting a VoIP service.

Referring to FIG. 3, the MAC apparatus 100 includes a frame handler 101,first and second transmission queues 102 and 103, a MAC controller 104,first and second arbitration inter-frame space (AIFS) timers 105 and106, first and second backoff blocks 107 and 108, a frame detector 109,a receive queue 110, and a collision resolution unit 111. Here, thefirst transmission queue 102 is composed of a voice frame transmissionfirst-in-first-out (FIFO), and the second transmission queue 103 iscomposed of a general frame transmission FIFO. The receive queue 110 isalso composed of a FIFO.

When a frame is received from an upper layer, the frame handler 101extracts user priority (UP) information from a traffic ID (TID) includedin a QoS control field (refer to FIG. 1) of a frame header, and if theframe is a voice frame, the frame handler 101 maps the frame to AC[3],and if the frame is a general frame, the frame handler 101 maps theframe to AC[0]-AC[2]. Also, if the frame is a voice frame, the framehandler 101 stores the frame in the first transmission queue 102, whichis the voice frame exclusive transmission FIFO, and if the frame is ageneral frame, the frame handler 101 stores the frame in the secondtransmission queue 103, which is the general frame transmission FIFO.

If it is determined by the frame handler 101 that the AC is 3 and avoice frame transmission request is generated, the MAC controller 104checks a medium status and determines a backoff operation timing and aframe transmission timing. Also, when the frame transmission request isgenerated, if the medium is busy, the MAC controller 104 waits until themedium is idle and sets the first and second AIFS timers 105 and 106 toan SIFS+AIFS[3] slot time according to the AC[3]. If the frame is avoice frame, a set value of the first AIFS timer 105 is the same as avalue of a priority inter-frame space (PIFS).

Each of the timers 105 and 106 is a timer for reducing the set value(SIFS+AIFS[3] slot time) in units of 1 μs. If the medium is still idleafter the timer value becomes 0, the MAC controller 104 commands arelevant backoff block to perform a backoff operation on the frame. Iftwo frames having different ACs are in the first and second transmissionqueues 102 and 103, respectively, and if separate frame transmissionrequests are generated, the MAC controller 104 commands the first andsecond backoff blocks 107 and 108 to independently perform backoffoperations on the two frames.

Each of the first and second backoff blocks 107 and 108 uses a pseudorandom integer evenly distributed in a range of [0, CW] as a backoffcount value. At this time, the CW is initialized with CWmin[AC] andincreases by CW=2(CW+1)−1 whenever frame transmission fails. The CW hasCWmax[AC] as a maximum value, and even if frame transmission fails, theCW does not increase more than CWmax[AC]. Each of the first and secondbackoff blocks 107 and 108 starts a backoff operation using a backoffcount value selected by the method described above, decreases a randombackoff count value in every slot time in which the medium is idle whileperforming the backoff operation, and informs the MAC controller 104 ofthe end of the backoff operation if the random backoff count valuebecomes 0.

If the MAC controller 104 is informed of the end of the backoffoperation, the MAC controller 104 transmits a frame of the AC for whichthe backoff operation is performed to a physical layer. At this time, iftwo ACs simultaneously end the backoff operations, the collisionresolution unit 111 transmits a voice frame having a higher UP first andgives up transmission of other general frames. The collision resolutionunit 111 commands the second backoff block 108 to perform the backoffoperation again using an increased CW value for the other general frame.

Also, when a transmission request is generated, if the medium is in awaiting status during the AFIS[AC] or performing the backoff operation,the MAC controller 104 waits until the medium is idle. When the mediumis idle, the MAC controller 104 sets one of the first and second AIFStimers 105 and 106 to an AIFS timer value according to an AC value andwaits until the AIFS timer value becomes 0. If the medium is still idleafter the AFIS[AC] time lapses, the MAC controller 104 starts a backoffoperation by selecting one of the first and second backoff blocks 107and 108. If the medium is idle during every slot time while performingthe backoff operation, the MAC controller 104 decreases a random backoffcount value. If the random backoff count value becomes 0, the MACcontroller 104 transmits a frame.

Also, when a transmission request is generated, if the medium is idlefor a longer time than the AFIS[AC] time, the MAC controller 104immediately transmits a frame.

When a frame is received from the physical layer, the frame detector 109determines whether or not to transmit an ACK frame by checking an ACKpolicy bit (refer to FIG. 1) included in the QoS control field of theframe header. However, the ACK frame is not transmitted if the receivedframe is a voice frame.

Also, if the received frame is a beacon frame transmitted from the AP,the frame detector 109 extracts parameters related to the QoS (Forexample, AIFS[AC], CWmin[AC], and CWmax[AC]) and updates existingvalues.

The receive queue 110 is composed of one FIFO, stores a frame, andtransmits a relevant frame to the upper layer whatever UPs of receivedframes are.

FIGS. 4 and 5 are flowcharts illustrating methods of supporting QoS in awireless LAN, which are performed in the MAC apparatus 100 shown in FIG.3, according to exemplary embodiments of the present invention. FIG. 4illustrates a processing method of the MAC apparatus 100, that isperformed in response to a frame received from an upper layer, and FIG.5 illustrates a processing method of the MAC apparatus 100, that isperformed in response to a frame received from a physical layer.

First, referring to FIG. 4, the MAC apparatus 100 determines a userpriority (UP) of a frame input from an upper layer and maps the frame toan access category (AC) in step 1000. The MAC apparatus 100 determineswhether the input frame is a voice frame in step 1010.

If the input frame is a voice frame corresponding to an AC[3] in step1010, the MAC apparatus 100 stores the frame in the first transmissionqueue 102, which is a voice exclusive FIFO, in step 1020. If the inputframe is a general frame corresponding to one of AC[0]-AC[2] in step1010, the MAC apparatus 100 stores the frame in the second transmissionqueue 103, which is a general FIFO, in step 1030.

After the voice frame is stored in the first transmission queue 102 instep 1020, it is determined whether a backoff operation has ended instep 1040, and if the backoff operation has not ended in step 1040, theMAC apparatus 100 performs the backoff operation of the voice frameusing CWmin[3], CWmax[3], and AIFS[3] in step 1050. It is determinedwhether a backoff operation has ended again in step 1040, and if thebackoff operation has ended, the MAC apparatus 100 performs step 1080.

On the other hand, after the general frame is stored in the secondtransmission queue 103 in step 1030, it is determined whether a backoffoperation has ended in step 1060, and if the backoff operation has notended in step 1060, the MAC apparatus 100 performs the backoff operationof the general frame using CWmin[AC], CWmax[AC], and AIFS[AC] accordingto the AC in step 1070. It is determined whether a backoff operation hasended again in step 1060, and if the backoff operation has ended, theMAC apparatus 100 performs step 1080.

It is determined whether the backoff operations of the voice frame andthe general frame have simultaneously ended in step 1080. If a singlebackoff operation ends in step 1080, the MAC apparatus 100 transmits theframe whose backoff operation ends regardless of priority in step 1090,and when the frame transmission ends, the MAC apparatus 100 switches toa receive mode in step 1110.

If it is determined that the backoff operations has simultaneously endedin step 1080, the MAC apparatus 100 performs internal collisionmanagement to which the priority is applied in step 1120. That is, theMAC apparatus 100 transmits the voice frame first by applying thepriority to the internal collision management in step 1100 and allowsthe backoff operation to be performed by increasing a backoff count instep 1070. When the frame transmission ends, the MAC apparatus 100switches to the receive mode in step 1110.

The processing method of the MAC apparatus 100 in response to a framereceived from a physical layer will now be described with reference toFIG. 5.

Referring to FIG. 5, the MAC apparatus 100 receives a frame from aphysical layer in step 1200. The MAC apparatus 100 decodes an ACK policyfrom a header of the received frame and determines whether an ACKresponse is performed using the ACK policy in step 1210. However, theACK response is not performed if the received frame is a voice frame. Ifthe ACK response must be performed as a result determined in step 1210,the MAC apparatus 100 transmits the ACK response in step 1220 and storesthe received frame in the receive queue 110 in step 1230. When a host isready, the MAC apparatus 100 transmits the frame to the upper layerregardless of priority in step 1240.

As described above, in a MAC apparatus and method for guaranteeing QoSin a wireless LAN according to embodiments of the present invention, fora real-time traffic VoIP such as an AC-3, a VoIP exclusive queue is used(refer to the reference number 102 of FIG. 3), and a back operation isperformed using AIFS[3], CWmin[3], and CWmax[3] parameters correspondingto the AC-3 to guarantee a higher priority. For simultaneouslygeneratable non-real-time traffic, a backoff operation is performedusing AIFS[AC], CWmin[AC], and CWmax[AC] parameters according to the ACto support all priorities except the VoIP traffic. Also, when the backoperations are simultaneously ended by the two transmission requeststhat are simultaneously generated, QoS is guaranteed by transmitting theVoIP traffic having a higher priority first.

The invention can also be embodied as computer readable codes on acomputer readable recording medium. The computer readable recordingmedium is any data storage device that can store data which can bethereafter read by a computer system. Examples of the computer readablerecording medium include read-only memory (ROM), random-access memory(RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storagedevices, and carrier waves (such as data transmission through theInternet). The computer readable recording medium can also bedistributed over network coupled computer systems so that the computerreadable code is stored and executed in a distributed fashion.

As described above, according to a MAC configuring method forguaranteeing QoS in a wireless LAN according to an embodiment of thepresent invention, a wireless LAN MAC, in which functions are simplifiedwhile guaranteeing QoS of a VoIP service, can be provided. Therefore, itbecomes easy to manufacture commercial chip with low costs, and aterminal supporting a wireless VoIP service with a low price can bemass-produced.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. A media access control (MAC) apparatus comprising: a firsttransmission queue exclusively storing a voice transmission frame; asecond transmission queue storing a general transmission frame and notstoring voice transmission frames; a frame handler extracting userpriority information from a frame input from an upper layer, mapping theframe to a relevant access category (AC), and storing the frame in thefirst transmission queue or the second transmission queue; a MACcontroller determining a backoff operation timing and a frametransmission timing by checking a media status; a first arbitrationinter-frame space (AIFS) timer and a second AIFS timer, each reducing apredetermined timer value set by the MAC controller by a predeterminedvalue unit; a first backoff block and a second backoff block, eachperforming a separate backoff operation for each AC using apredetermined backoff count value; a contention resolution unit, whichtransmits the voice frame having a higher user priority first and givesup transmission of the general frame when two ACs simultaneously end thebackoff operations; a frame detector, which determines whether or not totransmit an acknowledgment (ACK) frame by checking an ACK policy bitfrom a header of the received frame when a frame is received from aphysical layer; a receive queue storing the received frame andtransmitting the frame to the upper layer; and wherein the apparatusflexibly receives an ACK response based on the determination by theframe detector whether or not to transmit the ACK frame, but does notreceive the ACK response if the received frame is the voice frame,wherein the frame handler maps the received frame to an AC[3] when thereceived frame is a voice frame and to one of AC[0]-AC[2] when thereceived frame is a general frame, wherein the MAC controller performsi) the backoff operation by using an AIFS time (AIFS[3]) allocated tothe AC[3] and an initial value (CWmin[3]) and a maximum value (CWmax[3])of a contention window allocated to the AC[3] as Qos parameters forreal-time traffic including the voice frame, and ii) the backoffoperation by using an AIFS time (AIFS[AC]) allocated according to the ACand an initial value (CWmin[AC]) and a maximum value (CWmax[AC]) of acontention window allocated according to the AC as Qos parameters tosupport all priorities except the real-time traffic for non-real-timetraffic that can be simultaneously generated with the real-time traffic.2. The apparatus of claim 1, wherein the first transmission queue is avoice-over-IP (VOIP) exclusive first-in-first-out (FIFO) fortransmitting a voice frame, and the second transmission queue is ageneral FIFO for transmitting a general frame.
 3. The apparatus of claim1, wherein the MAC controller starts the backoff operation bycontrolling the first and second backoff blocks if the medium is in anidle or waiting status after a predetermined AIFS time lapses since atransmission request was generated, decreases the backoff count value inevery slot time in which the medium is in an idle status whileperforming the backoff operation, and ends the backoff operation if thebackoff count value becomes
 0. 4. The apparatus of claim 1, wherein eachof the first and second backoff blocks uses a pseudo random integeramong values evenly distributed between 0 and a contention window as thebackoff count value, and the contention window has an initial value anda maximum value defined according to the AC.
 5. A media access control(MAC) method comprising: extracting a user priority from a framereceived from an upper layer and storing a voice frame exclusively in afirst queue and separately storing a general frame in a second queueaccording to an access category (AC); independently performing backoffoperations for the voice frame and the general frame; determiningwhether the backoff operations for the voice frame and the general framehave simultaneously ended; if the backoff operations have simultaneouslyended, transmitting the voice frame having a higher priority first andperforming the backoff operation for the general frame; if the backoffoperations have not simultaneously ended, transmitting a frame whosebackoff operation ends; and wherein an acknowledgment (ACK) response isflexibly received based on a determination of whether or not to transmitan ACK frame, but the ACK response is not received if the received frameis the voice frame, checking an ACK policy bit from a header of thereceived frame when the frame is received from a physical layer; storingthe receive frame and transmitting the frame to the upper layer; mappingthe received frame to an AC[3] when the frame is a voice frame and toone of AC[0]-AC[2] when the received frame is a general frame, whereinthe MAC method performs i) the backoff operation by using an AIFS time(AIFS[3]) allocated to the AC[3] and an initial value (CWmin[3]) and amaximum value (CWmax[3]) of a contention window allocated to the AC[3]as Qos parameters for real-time traffic including the voice frame, andii) the backoff operation by using an AIFS time (AIFS[AC]) allocatedaccording to the AC and an initial value (CWmin[AC]) and a maximum value(CWmax[AC]) of a contention window allocated according to the AC as Qosparameters to support all priorities except the real-time traffic fornon-real-time traffic that can be simultaneously generated with thereal-time traffic.